IMO two aspects you must look at carefully are: the startup company, and the team. You obviously joined a startup because you believed in the founders, and what they are building. Does that still hold true? What kind of advisors and investors does your current company have on board, vs. what kind of advisors and investors are behind the other companies?

Also, more importantly, do you like working with you current team? Are you learning from them, and do you folks get along well? Not all teams are built the same, and there is a chance that you might not gel so well with team at your new company.

Also, in the long run, equity stake > base salary, for obvious reasons. If you truly believe in the company and want to work hard to make it grow or towards an exit, your additional equity stake could make all the difference. The way valuations are going these days, they will change pretty quickly.